Such devices have been known for several years and are used, for example, to inject a quantity measurement in internal combustion engines.
The original version of such a device for through-flow measurement was described in DE-AS 1 798 080. This electronically controlled flow meter comprises a main conduit with an inlet and an outlet, in which a rotary positive displacer in the form of a gear pump is arranged. A bypass runs parallel to the main conduit, via which bypass the rotary positive displacer can be bypassed and in which a piston serving as a differential pressure sensor is arranged in a measuring chamber. The excursion of the piston in the measuring chamber is measured using an optical sensor to determine the flow rate. The rotational speed of the gear pump is constantly readjusted via an evaluation and control unit based on this signal, the readjustment being such that the piston is always returned to its initial position, if possible, so that only small flows are generated in the bypass. The flow rate within a predefined time interval is calculated in this manner from the number of rotations or partial rotations of the gear pump measured by an encoder and from the known delivery quantity of the gear pump per revolution.
A flow quantity measuring device of this structure is also described in DE 103 31 228 B3. For determining the exact injection quantity profiles, the gear pump is set to a constant rotational speed prior to the start of each injection, so that the movement of the piston is measured subsequently, with this excursion being used to determine the injection profiles. A pressure sensor and a temperature sensor are also arranged in the measuring chamber, the measuring values of which are also supplied to the computing unit to calculate and to correct the injection quantity profiles.
It is also known to measure the excursion not via an optical sensor, but with a magneto-resistive sensor which corresponds with a permanent magnet arranged in the piston. Such a flow meter is described in AT 512619 B1.
In these flow quantity measuring devices use gear pumps which have two meshing gears arranged side by side, which gears are arranged in a pump chamber supplied with fluid via an inlet port, which fluid leaves the pump chamber via an outlet port. In order to exactly determine the quantity conveyed, as is required when used in flow meters, it is necessary, however, to be able to associate each revolution of the gears with an exact volume flow conveyed. Cavitation occurring in the inlet-side region as well as local pressure peaks in the region of the teeth in the outlet-side region and resulting wear may cause measuring errors during longer periods of operation.